My PRS-650 came with three dumb images that it cycles through in standby. I was going to go looking for new images when I fortuitously happened upon
nook-look.com. There are loads of images in a bunch of categories. And most of them are 600x800 pixels--just right.
Sure, the images are intended for use on a B&N
nook, but they work just great on my Sony.
Once you've copied them to your device, goto
Applications -> Pictures -> Options key -> Select Standby Screen
and select or unselect as desired and enjoy!